The invention generally relates to polymers employed in silica-filled vulcanizable elastomeric compositions. More particularly, the invention relates to stabilization of the viscosity of the polymers when exposed to moisture during desolventization and ambient storage conditions.
When producing polymers for use in rubber articles, such as tires, power belts, and the like, it is desirable that these polymers are easily processable during compounding and have a high molecular weight with a controlled molecular weight distribution, glass transition temperature (Tg) and vinyl content. It is also desirable that reinforcing fillers, such as silica and/or carbon black, be well dispersed throughout the rubber in order to improve various physical properties. Good dispersion of carbon black may be achieved, for example, by end-capping polydienes by reacting a living end of the polymer with an end-capping agent, or by utilizing functionalized anionic polymerization initiators such as lithium-based amine or amide initiators that incorporate a functional group onto one or both ends of the polymer chain. Rubber articles produced from vulcanized elastomers exhibiting these characteristics, will have reduced hysteresis resulting in an increase in rebound, a decrease in rolling resistance and less heat build-up when mechanical stresses are applied. These properties, in turn, result in lowered fuel consumption for vehicles using such tires.
With the increasing use of silica as a reinforcing filler for rubber, filler dispersion in rubber stocks has become a major concern. Because polar silanol groups on the surface of silica particles tend to self-associate, reagglomeration of silica particles occurs after compounding, leading to poor silica dispersion, a high compound viscosity and a shorter scorch time. One approach to achieving better dispersion of silica during compounding, disclosed in our co-pending, co-owned U.S. patent application, Ser. No. 08/891,569, involves termination of elastomeric polymers, such as diene rubbers, with a tin-containing coupling agent, such as tin tetrachloride, an organo-tin halide, a dialkyldioxastannylane compound, and the like, resulting in an increase in the Mooney viscosity of the gum polymer, which is desirable for better initial processability of the polymer. During compounding of the tin-functionalized polymers, the polymer carbon-tin bonds are cleaved, resulting in lower molecular weight fragments and, concomitantly, a lowered viscosity which allows better dispersability of the silica filler during compounding.
Another approach to improving dispersion of silica filler, involves modification of polymer chains with functional end groups that interact with or shield the surface hydroxyl groups on the silica filler. In particular, siloxane-terminated polymers are well known and their preparation is disclosed in U.S. Pat. Nos. 3,244,664 and 4,185,042. Siloxane-terminated polymers have a carbon-silicon bond and at least one terminal Oxe2x80x94R group that reacts with the silica surface, forming an Sixe2x80x94Oxe2x80x94Si linkage.
Although siloxane-terminated polymers have provided adequate dispersion of reinforcing fillers during compounding, there has been a problem with stabilizing the Mooney viscosity of the gum polymer prior to compounding. In particular, polymers produced by solution polymerization in inert organic solvents, such as hexane, require a desolventization step after polymerization. Although desolventization may be achieved by drum-drying, in commercial practice desolventization is achieved through the use of either steam or heated water. When the polymer chains are terminated by siloxane compounds, hydrolysis of pendant xe2x80x94Sixe2x80x94OR end groups during the desolventization step, leads to coupling of the polymer chains via formation of xe2x80x94Sixe2x80x94Oxe2x80x94Sixe2x80x94 bonds between two or more siloxane end groups, resulting in a large increase in the polymer molecular weight and, concomitantly, a large increase in the Mooney viscosity of the gum polymer. Moreover, during storage of siloxane-terminated polymers for a period of time prior to compounding, humid environmental conditions and residual water from desolventization can lead to further hydrolysis of end groups and polymer chain coupling, and a further increase in viscosity.
Several approaches have been taken to overcome this hydrolysis and coupling problem. For example, U.S. Pat. No. 5,659,056 discloses the use of acids such as C1 to C12 aliphatic and C6 to C12 cycloaliphatic and aromatic carboxylic acids including acetic acid, propionic acid, butyric acid, decanoic acid, cyclohexanoic acid, benzoic acid, and the like, as well as acyl halides thereof, as viscosity stabilizing agents to treat the polymer prior to desolventization. These viscosity stabilizing agents do not react with the siloxane terminal end groups of the polymer, but rather serve to neutralize the by-product lithium compounds in admixture with the polymer, thereby slowing the rate of formation of low boiling alcohols during desolventization, slowing the rate of the hydrolysis reaction and, therefore, slowing the rate of coupling of the polymer. In this manner, the increase in Mooney viscosity of the gum polymer during the steam or heated water desolventization step is substantially reduced.
Other approaches to controlling the Mooney viscosity of such siloxane-terminated polymers have included the use of alkyl alkoxysilanes, such as n-octyl triethoxysilane, as viscosity stabilizing agents. As disclosed in our co-owned, copending U.S. patent application Ser. No. 09/360,551, these viscosity stabilizing agents are also added prior to desolventization but, unlike the acids described above, they react with the siloxane-terminated polymers. Moreover, viscosity stabilizing agents such as alkyl alkoxysilanes eliminate, rather than just slow down, any increase in the Mooney viscosity for a period of time. The successful use of these viscosity stabilizing agents, however, is concentration dependent. That is, the number of xe2x80x94SiOR groups available from the addition of the viscosity stabilizing agent must be such that the majority of the Sixe2x80x94Oxe2x80x94Si bonds formed are between the hydrolyzable siloxane-terminated polymer and the viscosity stabilizing agent, not between the polymers themselves. Moreover, alkyl alkoxysilanes are relatively expensive compared to many other materials.
In another approach, our co-owned, copending U.S. patent application Ser. No. 09/449,303 discloses a method for stabilizing the Mooney viscosity of siloxane-terminated polymers, having at least one hydrolyzable constituent, by exchanging the alcohol of the siloxane terminal group for a long chain alcohol, such as an aliphatic, cycloaliphatic, or aromatic alcohol having at least 6 carbon atoms, or with a fatty acid ester of a hydrogenated or non-hydrogenated C5 or C6 sugar. The presence of the long chain alcohol or fatty acid ester sterically inhibits the availability of the hydrolyzable bond(s) to moisture. This approach results in slowing of the rate of coupling of the polymer which, in turn, slows the foreseen increase in Mooney viscosity of the siloxane-terminated polymer.
Although the above approaches to control of the Mooney viscosity of polymers having a hydrolyzable substituent have been shown to be successful, the need continues for alternative methods for controlling the rate of increase of Mooney viscosity of polymers prior to compounding, and also to provide a desirable lower viscosity during and after compounding for processability and adequate dispersion of the reinforcing fillers, especially silica.
The invention provides chain-coupled polymeric alkoxide compounds for use as high molecular weight polymers in vulcanizable elastomeric compositions comprising silica, carbon black, or mixtures of silica and carbon black as reinforcing fillers. The polymeric compounds of the invention are especially useful because of their processability when used in rubber making, i.e., they have an initial high molecular weight (high viscosity) at synthesis for ease of handling prior to compounding, and they are extremely resistant to any increase in viscosity due to the presence of moisture during desolventization or storage prior to compounding. During compounding, the viscosity of the polymers decreases as polymer chains become decoupled, to provide a reduced viscosity and improved interaction with the reinforcing filler for better filler dispersion. Moreover, the viscosity of the resulting compound provides for good processability during extrusion or molding of the compound at the tire plant.
In particular, the polymeric alkoxide compounds of the invention have the formula
(PAO)nM+zPxe2x80x2zxe2x88x92n
where P is a polymer chain; AO is an alkoxide group; Pxe2x80x2 is another polymer chain P or is an xe2x80x9cRxe2x80x9d group selected from the group consisting of alkyl groups having one to about 30 carbon atoms, aromatic groups having about 6 to about 20 carbon atoms, and cycloalkyl groups having about 5 to about 20 carbon atoms; M is a metal atom or a nonmetal atom, having an oxidation state xe2x80x9czxe2x80x9d of greater than one, wherein the nonmetal atom is selected from the group consisting of atoms of phosphorus, boron, nitrogen and sulfur; and n is an integer having a value of from 1 to z. Preferably the metal atom is selected from the group consisting of atoms of silicon, tin, titanium aluminum, arsenic, copper, calcium and zinc. For purposes of simplicity, silicon is considered herein to be a metal; however, one skilled in the art will understand that a silicon atom may act as either a metal or a nonmetal atom in the invention compounds.
The polymeric alkoxide compounds are produced by the process of reacting the living end of a polymer chain prepared by anionic polymerization with a monoalkyl metal initiator, after solution polymerization but while still in the presence of an inert solvent, with an alkoxide precursor compound selected from the group consisting of alkylene oxides and carbonyl compounds, and, subsequently, reacting the polymer chain with a coupling agent having the formula
M+zXzxe2x88x92mRm
where M+z and R are the same as above, X is a halide, (zxe2x88x92m) represents an integer having a value of at least 2, and m is an integer having a value of zero to (zxe2x88x922).
In one embodiment of the invention, the polymer chain having the living end is selected from the group consisting of homopolymers, copolymers and terpolymers of alkylene oxide monomers. For example, the monomers include, but are not limited to, ethylene oxide, propylene oxide, styrene oxide, cyclohexene oxide, cyclopentene oxide, and the like. In this embodiment of the invention, the step of reacting the living end of the polymer chain with an alkoxide precursor compound, prior to the reacting the polymer chain with the coupling agent, is omitted.
In another embodiment of the invention, the polymer chain having the living end is selected from the group consisting of homopolymers of conjugated diene monomers, and copolymers and terpolymers of the conjugated diene monomers with monovinyl aromatic monomers and trienes. Preferably, the polymer chain is selected from the group consisting of polyisoprene, polystyrene, polybutadiene, butadiene-isoprene copolymer, butadiene-isoprene-styrene terpolymer, isoprene-styrene copolymer, and styrene-butadiene copolymer. In another embodiment of the invention, one or more of these polymers are employed as the elastomeric component in a sulfur-vulcanizable elastomeric composition including a reinforcing filler selected from the group consisting of silica, carbon black, and mixtures thereof, and a cure agent. The invention further provides a pneumatic tire having at least one component produced from the vulcanizable elastomeric composition.
The polymeric alkoxide compound of the invention has an initial viscosity and, preferably, the viscosity of the compound does not increase above the initial viscosity by more than about 50%, more preferably not more than about 25%, and especially not more than about 10%, over a time period in storage of up to about two years, in ambient environmental conditions which may include hot, humid conditions.
The invention further provides a method for improving the mixing efficiency during compounding of an elastomer with a reinforcing filler, comprising the steps of providing a polymeric alkoxide compound having the formula described above; mixing the polymer in a mixer with a reinforcing filler selected from the group consisting of silica, carbon black, and mixtures thereof; providing a source of moisture; heating the mixture to a temperature of about 60xc2x0 C. to about 200xc2x0 C.; wherein during the mixing step up to xe2x80x9cnxe2x80x9d Oxe2x80x94M groups are hydrolyzed in the presence of the moisture and heat resulting in uncoupling of up to xe2x80x9cnxe2x80x9d polymer chains and a decrease in the viscosity of the mixture. Preferably, Pxe2x80x2 is also a P polymer chain and, during the mixing step, up to xe2x80x9czxe2x88x92nxe2x80x9d polymer chain carbon-M group bonds may be cleaved, resulting in a further decrease in the viscosity of the mixture. For example, cleavage of polymer carbon-M group bonds is known under these conditions when the M group is tin, lead, mercury or cadmium.
Due to hydrolysis of the Oxe2x80x94M groups, accompanied or unaccompanied by cleavage of polymer chain carbon-M groups, a vulcanizable elastomeric compound comprising the invention polymeric alkoxide compound has a viscosity that is reduced compared with the viscosity of an equivalent vulcanizable elastomeric compound comprising the same polymer, i.e., having the same monomer units, an equivalent molecular weight and initial degree of coupling, that does not contain either a hydrolyzable Oxe2x80x94M group or a cleavable carbon-M group.